Thermo-mechanical behaviors and moisture absorption of silica nanoparticle reinforcement in epoxy resins

An investigation of the thermo-mechanical behavior of silica nanoparticle reinforcement in two epoxy systems consisting of diglycidyl ether of bisphenol F (DGEBF) and cycloaliphatic epoxy resins was conducted. Silica nanoparticles with an average particle size of 20 nm were used. The mechanical and thermal properties, including coefficient of thermal expansion (CTE), modulus (E), thermal stability, fracture toughness (K_IC), and moisture absorption, were measured and compared against theoretical models. It was revealed that the thermal properties of the epoxy resins improved with silica nanoparticles, indicative of a lower CTE due to the much lower CTE of the fillers, and furthermore, DGEBF achieved even lower CTE than the cycloaliphatic system at the same wt.% filler content. Equally as important, the moduli of the epoxy systems were increased by the addition of the fillers due to the large surface contact created by the silica nanoparticles and the much higher modulus of the filler than the bulk polymer. In general, the measured values of CTE and modulus were in good agreement with the theoretical model predictions. With the Kerner and Halpin-Tsai models, however, a slight deviation was observed at high wt.% of fillers. The addition of silica nanoparticles resulted in an undesirable reduction of glass transition temperature (T_g) of approximately 20 °C for the DGEBF system, however, the T_g was found to increase and improve for the cycloaliphatic system with silica nanoparticles by approximately 16 °C. Furthermore, the thermal stability improved with addition of silica nanoparticles where the decomposition temperature (T_d) increased by 10 °C for the DGEBF system and the char yield significantly improved at 600 °C. The moisture absorption was also reduced for both DGEBF and cycloaliphatic epoxies with filler content. Lastly, the highest fracture toughness was achieved with approximately 20 wt.% and 15 wt.% of silica nanoparticles in DGEBF and cycloaliphatic epoxy resins, respectively.     

Toughening of epoxy hybrid nanocomposites modified with silica nanoparticles and epoxidized natural rubber

Silica nanoparticles (SN) and epoxidized natural rubber (ENR) were used as binary component fillers in toughening diglycidyl ether of bisphenol A (DGEBA) cured cycloaliphatic polyamine. For a single component filler system, the addition of ENR resulted in significantly improved fracture toughness (K_IC) but reduction of glass transition temperature (T_g) and modulus of epoxy resins. On the other hand, the addition of SN resulted in a modest increase in toughness and T_g but significant improvement in modulus. Combining and balancing both fillers in hybrid ENR/SN/epoxy systems exhibited improvements in the Young’s modulus and T_g, and most importantly the K_IC, which can be explained by synergistic impact from the inherent characteristics associated with each filler. The highest K_IC was achieved with addition of small amounts of SN (5 wt.%) to the epoxy containing 5–7.5 wt.% ENR, where the K_IC was distinctly higher than with the epoxy containing ENR alone at the same total filler content. Evidence through scanning electron microscopy (SEM) and transmission optical microscopy (TOM) revealed that cavitation of rubber particles with matrix shear yielding and particle debonding with subsequent void growth of silica nanoparticles were the main toughening mechanisms for the toughness improvements for epoxy. The fracture toughness enhancement for hybrid nanocomposites involved an increase in damage zone size in epoxy matrix due to the presence of ENR and SN, which led to dissipating more energy near the crack-tip region.     

The mechanisms and mechanics of the toughening of epoxy polymers modified with silica nanoparticles

The present paper considers the mechanical and fracture properties of four different epoxy polymers containing 0, 10 and 20 wt.% of well-dispersed silica nanoparticles. Firstly, it was found that, for any given epoxy polymer, their Young’s modulus steadily increased as the volume fraction, v_f, of the silica nanoparticles was increased. Modelling studies showed that the measured moduli of the different silica-nanoparticle filled epoxy polymers lay between upper-bound values set by the Halpin-Tsai and the Nielsen ‘no-slip’ models, and lower-bound values set by the Nielsen ‘slip’ model; with the last model being the more accurate at relatively high values of v_f. Secondly, the presence of silica nanoparticles always led to an increase in the toughness of the epoxy polymer. However, to what extent a given epoxy polymer could be so toughened was related to structure/property relationships which were governed by (a) the values of glass transition temperature, T_g, and molecular weight, M_c, between cross-links of the epoxy polymer, and (b) the adhesion acting at the silica nanoparticle/epoxy-polymer interface. Thirdly, the two toughening mechanisms which were operative in all the epoxy polymers containing silica nanoparticles were identified to be (a) localised shear bands initiated by the stress concentrations around the periphery of the silica nanoparticles, and (b) debonding of the silica nanoparticles followed by subsequent plastic void growth of the epoxy polymer. Finally, the toughening mechanisms have been quantitatively modelled and there was good agreement between the experimentally-measured values and the predicted values of the fracture energy, G_c, for all the epoxy polymers modified by the presence of silica nanoparticles. The modelling studies have emphasised the important roles of the stress versus strain behaviour of the epoxy polymer and the silica nanoparticle/epoxy-polymer interfacial adhesion in influencing the extent of the two toughening mechanisms, and hence the overall fracture energy, G_c, of the nanoparticle-filled polymers.     

The effect of silica nanoparticles and carbon nanotubes on the toughness of a thermosetting epoxy polymer

Silica nanoparticles and multiwalled carbon nanotubes (MWCNTs) have been incorporated into an anhydride‐cured epoxy resin to form “hybrid” nanocomposites. A good dispersion of the silica nanoparticles was found to occur, even at relatively high concentrations of the nanoparticles. However, in contrast, the MWCNTs were not so well dispersed but relatively agglomerated. The glass transition temperature of the epoxy polymer was 145°C and was not significantly affected by the addition of the silica nanoparticles or the MWCNTs. The Young's modulus was increased by the addition of the silica nanoparticles, but the addition of up to 0.18 wt % MWCNTs had no further significant effect. The addition of both MWCNTs and silica nanoparticles led to a significant improvement in the fracture toughness of these polymeric nanocomposites. For example, the fracture toughness was increased from 0.69 MPam^1/2 for the unmodified epoxy polymer to 1.03 MPam^1/2 for the hybrid nanocomposite containing both 0.18 wt % MWCNTs and 6.0 wt % silica nanoparticles; the fracture energy was also increased from 133 to 204 J/m². The mechanisms responsible for the enhancements in the measured toughness were identified by observing the fracture surfaces using field‐emission gun scanning electron microscopy. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Combined numerical/experimental investigation of particle diameter and interphase effects on coefficient of thermal expansion and young's modulus of SiO2/epoxy nanocomposites

A combined numerical/experimental approach is used to study the effects of the particle/matrix interphase on the coefficient of thermal expansion (CTE) and Young's modulus of SiO₂/epoxy nanocomposites having nanoparticle reinforcements of different sizes. Our experiments showed that the composite CTE decreases and composite Young's modulus increases with decreasing nanoparticle diameter at the same volume fraction, but our finite element (FE) model predictions did not match the expected trends when the interphase was not accounted for. The new models include an interphase region around the nanoparticle which results in an “effective particle volume fraction” that is larger than the actual particle volume fraction. The results from the models are compared with the experimental results and the new models are accurately fitted to the experimental results using the interphase thickness as a curve‐fitting parameter. We believe that this is the first published report on the use of combined numerical/experimental investigations of both elastic stiffness and thermal expansion characteristics to demonstrate the existence of a particle size‐dependent “effective particle volume fraction” due to the particle/matrix interphase region in a nanoparticle‐reinforced composite. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers     

Toughening epoxies with halloysite nanotubes

An experimental attempt was made to characterize the fracture behaviour of epoxies modified by halloysite nanotubes and to investigate toughening mechanisms with nanoparticles other than carbon nanotubes (CNTs) and montmorillonite particles (MMTs). Halloysite-epoxy nanocomposites were prepared by mixing epoxy resin with halloysite particles (5 wt% and 10 wt%, respectively). It was found that halloysite nanoparticles, mainly nanotubes, are effective additives in increasing the fracture toughness of epoxy resins without sacrificing other properties such as strength, modulus and glass transition temperature. Indeed, there were also noticeable enhancements in strength and modulus for halloysite-epoxy nanocomposites because of the reinforcing effect of the halloysite nanotubes due to their large aspect ratios. Fracture toughness of the halloysite particle modified epoxies was markedly increased with the greatest improvement up to 50% in K_IC and 127% in G_IC. Increases in fracture toughness are mainly due to mechanisms such as crack bridging, crack deflection and plastic deformation of the epoxy around the halloysite particle clusters. Halloysite particle clusters can interact with cracks at the crack front, resisting the advance of the crack and resulting in an increase in fracture toughness.     

The modelling of the toughening of epoxy polymers via silica nanoparticles: The effects of volume fraction and particle size

Silica nanoparticles possessing three different diameters (23, 74 and 170 nm) were used to modify a piperidine-cured epoxy polymer. Fracture tests were performed and values of the toughness increased steadily as the concentration of silica nanoparticles was increased. However, no significant effects of particle size were found on the measured value of toughness. The toughening mechanisms were identified as (i) the formation of localised shear-band yielding in the epoxy matrix polymer which is initiated by the silica nanoparticles, and (ii) debonding of the silica nanoparticles followed by plastic void growth of the epoxy matrix polymer. These mechanisms, and hence the toughness of the epoxy polymers containing the silica nanoparticles, were modelled using the Hsieh et al. approach (Polymer 51, 2010, 6284–6294). However, it is noteworthy that previous modelling work has required the volume fraction of debonded silica particles to be measured from the fracture surfaces but in the present paper a new and more fundamental approach has been proposed. Here finite-element modelling has demonstrated that once one silica nanoparticle debonds then its nearest neighbours are shielded from the applied stress field, and hence may not debond. Statistical analysis showed that, for a good, i.e. random, dispersion of nanoparticles, each nanoparticle has six nearest neighbours, so only one in seven particles would be predicted to debond. This approach therefore predicts that only 14.3% of the nanoparticles present will debond, and this value is in excellent agreement with the value of 10–15% of those nanoparticles present debonding which was recorded via direct observations of the fracture surfaces. Further, this value of about 15% of silica nanoparticles particles present debonding has also been noted in other published studies, but has never been previously explained. The predictions from the modelling studies of the toughness of the various epoxy polymers containing the silica nanoparticles were compared with the measured fracture energies and the agreement was found to be good.     

Study of epoxy toughened by in situ formed rubber nanoparticles

The effect of rubber nanoparticles on mechanical properties and fracture toughness was investigated. Rubber nanoparticles of 2–3 nm were in situ synthesized in epoxy taking advantage of the reaction of an oligomer diamine with epoxy. The chemical reaction was verified by gel permeation chromatography (GPC) and ¹HNMR, and the microstructure was characterized by transmission electron microscope. The rubber nanoparticles caused much less Young's modulus deterioration but toughened epoxy to a similar degree in comparison with their peer liquid rubber that formed microscale particles during curing. Fifteen wt % of rubber nanoparticles increased fracture energy from 140 to 840 J/m² with Young's modulus loss from 2.85 to 2.49 GPa. The toughening mechanism might be the stress relaxation of the matrix epoxy leading to larger plastic work absorbed at the crack tip; there is no particle cavitation or deformation; neither crack deflection nor particle bridging were observed. The compound containing rubber nanoparticles demonstrates Newtonian liquid behavior with increasing shear rate; it shows lower initial viscosity at low shear rate than neat epoxy; this provides supplementary evidence to NMR and GPC result. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Mild processing and characterization of silica epoxy hybrid nanocomposite

Previous research has shown that the inclusion of the spherical silica (SiO₂) nanoparticles into epoxy resin can achieve simultaneous improvement of fracture toughness and modulus. However, the glass transition temperature of the nanocomposite was significantly decreased when loading the nanosilica was higher than 5 wt.%. This perhaps was caused by utilization of the ultrasonication probe in the processing of these materials. In this paper, milder processing procedures were applied to make spherical silica epoxy nanocomposites while investigating if the homogeneous dispersion and morphology of the individual silica nanoparticle dispersed in the epoxy matrix could still be achieved. The results show that even at high loading of the silica nanoparticle, such as 30 wt.% silica, the perfect morphology of the nanocomposite could still be achieved with these milder processing conditions which indicates that ultrasonication is not needed. With the use of milder processing conditions, the glass transition temperature of the nanocomposite of 5 wt.% silica loading did not change, and the drop in the T_g was minimal for silica loading up to 15%, but some effects of self-polymerization of the epoxy were noted on T_g up to 30 wt.% loading of silica. Thermal analysis and flammability testing of the resulting materials suggest that nanosilica has only an inert filler effect (dilution of fuel) on flammability reduction and char yield increase, not a synergistic decrease in heat release as is often observed for clays and carbon nanotubes/nanofibers. So the mild and easy processing procedure only achieved uniform nanoscale morphology with excellent dispersion in the final nanocomposite, but also the effect on the change in the T_g can be minimized as nanosilica loading was increased.     

Effect of single‐mineral filler and hybrid‐mineral filler additives on the properties of polypropylene composites

The present study was carried out to determine the filler characteristics and to investigate the effects of three types of mineral fillers (CaCO₃, silica, and mica) and filler loadings (10–40 wt%) on the properties of polypropylene (PP) composites. The characteristics of the particulate fillers, such as mean particle size, particle size distribution, aspect ratio, shape, and degree of crystallinity were identified. In terms of mechanical properties, for all of the filled PP composites, Young's modulus increased, whereas tensile strength and strain at break decreased as the filler loading increased. However, 10 wt% of mica in a PP composite showed a tensile strength comparable with that of unfilled PP. Greater tensile strength of mica/PP composites compared to that of the other composites was observed because of lower percentages of voids and a higher aspect ratio of the filler. Mica/PP also exhibited a lower coefficient of thermal expansion (CTE) compared to that of the other composites. This difference was due to a lower degree of crystallinity of the filler and the CTE value of the mica filler. Scanning electron microscopy was used to examine the structure of fracture surfaces, and there was a gradual change in tensile fracture behavior from ductile to brittle as the filler loading increased. The nucleating ability of the fillers was studied with differential scanning calorimetry, and a drop in crystallinity of the composites was observed with the addition of mineral filler. Studies on the hybridization effect of different (silica and mica) filler ratios on the properties of PP hybrid composites showed that the addition of mica to silica‐PP composites enhanced their tensile strength and modulus. J. VINYL ADDIT. TECHNOL., 2009. © 2009 Society of Plastics Engineers     

Structure-property relationships in rubber-toughened epoxies

Epoxies toughened with two reactive liquid rubbers, an epoxy-terminated butadiene acrylonitrile rubber (ETBN) and an amino-terminated butadiene acrylonitrile rubber (ATBN), were prepared and studied in terms of their structure property relationships. A two-phase structure was formed, consisting of spherical rubber particles dispersed in an epoxy matrix. A broad distribution of rubber particles was observed in all the materials with most of the particles ranging in size from 1 to 4 μm, but some particles exceeding 20 μm were also found. Impact strength, plane strain fracture toughness (K_IC), and fracture energy (G_IC) were increased, while Young's modulus and yield strength decreased slightly with increasing rubber content and volume fraction of the dispersed phase. Both G_IC and K_IC were found to increase with increasing apparent molecular weight between crosslinks and decreasing yield strength. The increased size of the plastic zone at the crack tip associated with decreasing yield strength could be the cause of the increased toughness. An ATBN-toughened system containing the greatest amount of epoxy sub-inclusion in the rubbery phase demonstrated the best fracture toughness in this series. In the present systems, rubber-enhanced shear deformation of the matrix is considered to be the major toughening mechanism. Curing conditions and the miscibility between the liquid rubber and the epoxy resin determine the phase morphology of the resulting two-phase systems. Kerner's equation successfully describes the modulus dependence on volume fraction for the two-phase epoxy materials.     

Feasibility research of Yb3Al5O12 garnet as environmental barrier coating materials

In this work, Yb₃Al₅O₁₂ (YbAG) garnet, as a new material for environment barrier coating (EBC) application, was synthesized and prepared by atmospheric plasma spraying (APS). The phases and microstructures of the coatings were characterized by XRD, EDS and SEM, respectively. The thermal stability was measured by TG-DSC. The mechanical and thermal-physical properties, including Vickers hardness (H_v), fracture toughness (K_IC), Young's modulus (E), thermal conductivity (κ) and coefficient of thermal expansion (CTE) were also measured. The results showed that the as-sprayed coating was mainly composed of crystalline Yb₃Al₅O₁₂ and amorphous phase which crystallized at around 917 °C. Moreover, it has a hardness of 6.81 ± 0.23 GPa, fracture toughness of 1.61 ± 0.18 MPa m^1/2, as well as low thermal conductivity (0.82–1.37 W/m·K from RT-1000 °C) and an average coefficient of thermal expansion (CTE) (∼6.3 × 10⁻⁶ K⁻¹ from RT to 660 °C). In addition, the thermal shock and water-vapor corrosion behaviors of the Yb₃Al₅O₁₂-EBC systems on the SiC_f/SiC substrates were investigated and their failure mechanisms were analyzed in details. The Yb₃Al₅O₁₂ coating has an average thermal shock lifetime of 72 ± 10 cycles as well as an excellent resistance to steam. These combined properties indicated that the Yb₃Al₅O₁₂ coating might be a potential EBC material. Both the thermal shock failure and the steam recession of the Yb₃Al₅O₁₂-EBC systems are primarily associated with the CTE mismatch stress.     

Fabrication and mechanical properties of Al2O3-SiCw-TiCnp ceramic tool material

Whiskers and nanoparticles are usually used as reinforcing additives of ceramic composite materials due to the synergistically toughening and strengthening mechanisms. In this paper, the effects of TiC nanoparticle content, particle size and preparation process on the mechanical properties of hot pressed Al₂O₃-SiC_w ceramic tool materials were investigated. The results showed that the Vickers hardness and fracture toughness of the materials increased with the increasing of TiC content. The optimized flexural strength was obtained with TiC content of 4 vol% and particle size of 40 nm. The particle size has been found to have a great influence on flexural strength and small influence on hardness and fracture toughness. It was concluded that the flexural strength increased remarkably with the decreasing of the TiC particle size, which was resulted from the improved density and refined grain size of the composite material due to the dispersion of the smaller TiC particle size. SEM micrographs of fracture surface showed the whiskers to be mainly distributed along the direction perpendicular to the hot-pressing direction. The fracture toughness was improved by whisker crack bridging, crack deflection and whisker pullout; the TiC nanoparticles in Al₂O₃ grains caused transgranular fracture and crack deflection, which improved the flexural strength and fracture toughness with whiskers synergistically. Uniaxial hot-pressing of SiC whisker reinforced Al₂O₃ ceramic composites resulted in the anisotropy of whiskers’ distribution, which led to crack propagation differences between lateral crack and radical crack.     

Toughening mechanisms of nanoparticle-modified epoxy polymers

An epoxy resin, cured with an anhydride, has been modified by the addition of silica nanoparticles. The particles were introduced via a sol-gel technique which gave a very well-dispersed phase of nanosilica particles which were about 20 nm in diameter. Atomic force and electron microscopies showed that the nanoparticles were well-dispersed throughout the epoxy matrix. The glass transition temperature was unchanged by the addition of the nanoparticles, but both the modulus and toughness were increased. The measured modulus was compared to theoretical models, and good agreement was found. The fracture energy increased from 100 J/m² for the unmodified epoxy polymer to 460 J/m² for the epoxy polymer with 13 vol% of nanosilica. The fracture surfaces were inspected using scanning electron and atomic force microscopies, and the results were compared to various toughening mechanisms proposed in the literature. The toughening mechanisms of crack pinning, crack deflection and immobilised polymer were discounted. The microscopy showed evidence of debonding of the nanoparticles and subsequent plastic void growth. A theoretical model of plastic void growth was used to confirm that this mechanism was indeed most likely to be responsible for the increased toughness that was observed due to the presence of the nanoparticles.     

Relations between the structure and the mechanical properties of fluidized-bed pyrolytic carbons

The mechanical properties of fluidized-bed pyrolytic carbons derived from a number of hydrocarbon gases have been related to the density, apparent crystallite size, and degree of preferred orientation. For isotropic carbons with constant crystallite size, the Young's modulus and the fracture stress increase with increasing density. Over a considerable range of density, the Young's modulus follows the relation E = E₀ exp (−BP) where E₀ and B are constants and P is the fractional porosity. Over a similar range of density the increase in fracture stress is apparently related to the changing volume fraction of the pores and not to changes in the critical flaw size or the work of fracture. For isotropic carbons with constant density, the Young's modulus and fracture stress increase with decreasing crystallite size possibly due to cross-linking either between crystallites or between layer planes. For carbons with constant density and crystallite size, the Young's modulus increases and the fracture stress decreases with increasing anisotropy. The variation of the Young's modulus shows fair agreement with the variation expected from constant-stress crystallite averaging while the variation of the fracture stress might be explained by a change in the mode of fracture propagation.     

Toughening of Epoxy Adhesives by Nanoparticles

With the emergence and commercialization of nanoparticles, new opportunities have emerged for toughening of epoxy adhesives using nanoparticles without sacrificing strength, rigidity and glass transition temperature, as is the case with conventional elastomeric tougheners. Inorganic Fullerene-like tungsten disulfide (IF-WS₂) nanoparticles and functionalized nano-POSS (Polyhedral-Oligomeric-Sil-Sesquioxane) were used to study the effects of nanoparticles on the toughening and mechanical properties of low and high temperature curing epoxy systems. Experimental results indicated that IF-WS₂ increased the fracture toughness by more than 10 fold in both epoxy systems at very low concentrations (0.3–0.5 wt%) while increasing its storage modulus and preserving its glass transition temperature. Epoxy functionalized POSS demonstrated an increase in toughness in addition to preserving rigidity and thermal properties at higher concentrations (3 wt%). It was postulated that chemical interaction of the sulfide and the epoxy matrix and the inherent properties of WS₂ were the decisive factors with respect to the outstanding nano-effect in the case IF-WS₂.     

Control of the functionality of graphene oxide for its application in epoxy nanocomposites

Amino- and epoxy-functionalized graphene oxide (GO) were synthesized separately through a wash-and-rebuild process utilizing two differently terminated silane coupling agents. The modified GO sheets were then incorporated into an epoxy resin to prepare nanocomposites. The addition of 0.2 wt% amino-functionalized GO (APTS-GO) yielded a 32% increase in Young's modulus (3.3 GPa) and 16% increase in tensile strength (81.2 MPa). Less reinforcement was observed with the epoxy-functionalized GO (GPTS-GO) but there was a more significant increase in ductility for GPTS-GO/epoxy, with the fracture toughness (critical intensity factor, K_IC) and fracture energy (critical strain energy release rate, G_IC) nearly doubling at 0.2 wt% loading (1.46 MPam^1/2 and 0.62 kJ/m² for K_IC and G_IC, respectively). Raman spectroscopy measurements revealed that the GPTS-GO was dispersed more uniformly than the APTS-GO in the epoxy matrix, and better interfacial stress transfer was found for the APTS-GO. Thus the wash-and-rebuild process affords a novel strategy for controlling the functionality of graphene in the quest to develop high-performance graphene-based nanocomposites.     

Mechanical and thermal shock properties of size graded MgO–PSZ refractory

The effects of the mixture of coarse powder with fine PSZ powder on the thermal-mechanical properties of 10 Mg–PSZ samples were studied. The size graded specimens were injection-molded using 3.5 m% MgO–ZrO₂ powders. The physical properties of the ZrO₂ samples and five thermal shock parameters were measured and calculated. These properties included density (ρ), porosity (p), the ratio of m/(t+c+m) phase, fracture toughness (K_IC), strength (σ_f), Young's modulus (E), shear modulus (G), Poisson's ratio (ν), and the thermal expansion (α) between ambient temperature to 1100°C. The toughness and thermal shock resistance of the PSZ are controlled by the states of porous microstructure which can be represented by a parameter (nominal largest tolerable length of defects) a_t. The PSZ samples show two types of thermal shock behavior differentiated by comparing the value of a_t to the characteristic length L_f of the defects in the sintered PSZ. The states of the defects, i.e. porosity, are the microstructural evidence to explain the relationship between the thermal shock properties.     

The effect of the interface structure of different surface‐modified nano‐SiO2 on the mechanical properties of nylon 66 composites

The nylon 66‐based nanocomposites containing two different surface‐modified and unmodified SiO₂ nanoparticles were prepared by melt compounding. The interface structure formed in different composite system and their influences on material mechanical properties were investigated. The results indicated that the interfacial interactions differed between composite systems. The strong interfacial adhesion helped to increase tensile strength and elastic modulus of composites; whereas, the presence of modification layer in silica surface could enhance the toughness of composites, but the improvement of final material toughness was also correlated with the density of the adhered nylon 66 chains around silica nanoparticles. In addition, the results also indicated that the addition of surface‐modified silica nanoparticles has a distinct influence on the nonisothermal crystallization behavior of the nylon 66 matrix when compared with the unmodified silica nanoparticle. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Highly dispersed nanosilica-epoxy resins with enhanced mechanical properties

Epoxy-nanocomposite resins filled with 12-nm spherical silica particles were investigated for their thermal and mechanical properties as a function of silica loading. The nanoparticles were easily dispersed with minimal aggregation for loadings up to 25 wt% as determined using transmission electron microscopy (TEM) and ultra-small-angle X-ray scattering (USAXS). A proportional decrease in cure temperatures and glass transition temperature (for loadings of 10 wt% and above) was observed with increased silica loading. The morphology determined by USAXS is consistent with a zone around the silica particles from which neighboring particles are excluded. The “exclusion zone” extends to 10× the particle diameter. For samples with loadings less than 10 wt%, increases of 25% in tensile modulus and 30% in fracture toughness were obtained. More highly loaded samples continued to increase in modulus, but decreased in strength and fracture toughness. Overall, the addition of nanosilica is shown as a promising method for property enhancement of aerospace epoxy composite resins.